threads: enable access to pthread_attr_get functions (WebAssembly#357)

The pthreads API exposes functions for querying the attributes of a
thread. This change allows these functions to be compiled in the
`THREAD_MODEL=posix` build. Some functions are skipped (and documented);
they can be added if/when needed. This change is motivated by a
`libc-test` test that uses these functions.

Author: abrown

Makefile

@@ -228,6 +228,7 @@ LIBC_TOP_HALF_MUSL_SOURCES += \
         thread/__timedwait.c \
         thread/default_attr.c \
         thread/pthread_attr_destroy.c \
+        thread/pthread_attr_get.c \
         thread/pthread_attr_init.c \
         thread/pthread_attr_setstack.c \
         thread/pthread_attr_setstacksize.c \

libc-top-half/musl/src/thread/pthread_attr_get.c

@@ -17,6 +17,7 @@ int pthread_attr_getinheritsched(const pthread_attr_t *restrict a, int *restrict
 	return 0;
 }
 
+#ifdef __wasilibc_unmodified_upstream /* WASI has no CPU scheduling support. */
 int pthread_attr_getschedparam(const pthread_attr_t *restrict a, struct sched_param *restrict param)
 {
 	param->sched_priority = a->_a_prio;
@@ -28,6 +29,7 @@ int pthread_attr_getschedpolicy(const pthread_attr_t *restrict a, int *restrict
 	*policy = a->_a_policy;
 	return 0;
 }
+#endif
 
 int pthread_attr_getscope(const pthread_attr_t *restrict a, int *restrict scope)
 {
@@ -56,6 +58,7 @@ int pthread_barrierattr_getpshared(const pthread_barrierattr_t *restrict a, int
 	return 0;
 }
 
+#ifdef __wasilibc_unmodified_upstream /* Forward declaration of WASI's `__clockid` type. */
 int pthread_condattr_getclock(const pthread_condattr_t *restrict a, clockid_t *restrict clk)
 {
 #ifdef __wasilibc_unmodified_upstream
@@ -65,6 +68,7 @@ int pthread_condattr_getclock(const pthread_condattr_t *restrict a, clockid_t *r
 #endif
 	return 0;
 }
+#endif
 
 int pthread_condattr_getpshared(const pthread_condattr_t *restrict a, int *restrict pshared)
 {